apple TV low quality output

Hi,

I have an HD Apple TV device, late 2012 or early 2013 I don't quite remember.
Whenever I mirror my iMac screen on the TV screen the quality is bad.
Even when I play HD files on the iMac from YouTube or whatever they don't appear on the TV as well as they appear at the same time on the computer...
Not just HD but everything is low quality compared to what's appearing on the computer.
I've tried playing with the settings but everything seems to be ok, the output is set on 1080p and all that..

Is it supposed to be this way?

If you are playing some content directly from Apple TV throughout to your TV do you notice similar quality drops? If not, and the difference is only noticeable in compared to the computer, I suspect you're just seeing a difference between the resolution of the computer and TV. The Macs typically produce much higher resolution than TVs.